Return Temperature Research Articles

Impact of Domestic Hot Water Systems on District Heating Temperatures

When buildings become more energy effective, the temperature levels of district heating systems need to be lower to decrease the losses from the distribution system and to keep district heating a competitive alternative on the heating market. For this reason, buildings that are refurbished need to be adapted to suit low-temperature district heating. The aim of this paper is to examine whether four different energy refurbishment packages (ERPs) can be used for lowering the temperature need of a multi-family buildings space heating and domestic hot water (DHW) system as well as to analyse the impact of the DHW circulation system on the return temperature. The results show that for all ERPs examined in this study, the space heating supply temperature agreed well with the temperature levels of a low-temperature district heating system. The results show that the temperature need of the DHW system will determine the supply temperature of the district heating system. In addition, the amount of days with heating demand decreases for all ERPs, which further increases the influence of the DHW system on the district heating system. In conclusion, the DHW system needs to be improved to enable the temperature levels of a low-temperature district heating system.

Determining the optimum low-temperature district heating network design for a secondary network supplying a low-energy-use apartment block in Ireland

District heating is an efficient method of delivering thermal energy to buildings. The latest, 4th generaton of the technology, can be applied to integrate multiple renewable and low-carbon energy sources into the thermal energy mix. Domestic heat demand in Ireland is heavily reliant on fossil fuels, prominently oil and gas. Despite the proven benefits of district heating, it has been underutilized in Ireland. Opportunities lie for the implementation of district heating networks alongside newly built buildings in Ireland. This study determines the cost-optimum design of a secondary district heating network, serving an Irish apartment block, built to the most recent building energy standards. Supply temperature, return temperature, and pipe diameter are varied to minimise lifetime cost of the network. Mathematical modelling is applied, through MATLAB software, to achieve this cost-optimisation. 1 × 1010 possible pipe configurations are examined for a range of thirty different supply and return temperatures. The optimum supply and return temperatures are calculated to be 61 °C and 27 °C respectively.

Semi-parametric modelling of sun position dependent solar gain using B-splines in grey-box models

Modelling the effects of solar irradiation plays an important role in various applications. This paper describes a semi-parametric (combined grey-box and spline-based), data-driven technique that can be used to model systems in which the solar gain depends on the sun position. The solar gain factor is introduced, i.e. the absorbed fraction of measured solar irradiation, and estimated as a continuous non-parametric function of the sun position. The implementation of the spline-based solar gain factor in a grey-box model framework is described. The method is tested in two case studies—in a model of the internal temperature of a dwelling in Aalborg, Denmark, and a model of the return temperature of a solar collector field in Solrød, Denmark. It is shown that the solar gain factor as a function of sun position is able to account for structural variations in solar gain that may occur due to factors such as shading obstacles and window or absorber orientation. In both test cases, the spline-based solar gain function improved the model accuracy significantly, and largely reduced structural errors in prediction residuals. In addition, the shape of the estimated function provided insight into the dynamics of the system and the local solar input characteristics. Accurate representation of such site characteristics was not possible with any data-driven method found in the literature. Besides the grey-box models used in this study, the solar gain factor can be used in a variety of data-driven models, for example in linear regression models.

High temperature heat and water recovery in steam injected gas turbines using an open absorption heat pump

The advantages of reinjecting steam from the heat recovery steam generator (HRSG) in the preceding gas turbine are increased power and electrical efficiency at low specific cost and a high operating flexibility. The discharge of the injected steam to the ambient has however two major drawbacks: (1) a relevant water consumption and (2) a large thermal loss related to the latent heat of steam. An open absorption heat pump cycle downstream of the HRSG can solve both problems, as it allows to recover the steam from the flue gas and use its condensation heat at elevated temperature. This paper presents for the first time a concept to efficiently integrate both technologies and assesses the potential and limitations of the absorption system for a steam injected gas turbine. For high network return temperatures (60 °C), the “High Temperature Condensation Boiler Technology” (HT-CBT) performs significantly better than a conventional flue gas condenser over the entire STIG operating range, both with respect to fuel efficiency and water recovery. For steam injection rates up to 1.16 kg/s (60 °C) and 0.96 kg/s (40 °C) fuel efficiency remains constant at ca. 95% resp. 102% and power can hence be increased without the associated energetic penalty. Within this range, full water recovery eliminates the high water usage of the Cheng Cycle, making it a suited technology for flexible cogeneration also in dry areas. For higher steam injection rates, the added benefit of the HT-CBT was found to be restricted by the limited availability of desorber heating from the HRSG, suggesting the integration of external desorber heating. The possibility to increase the power output for a given heat demand, with limited impact on fuel efficiency, increases the economic viability of the technology compared to heating only applications.

Multi-mode control method for the existing domestic hot water storage tanks with district heating supply

The hot water tank is widely used for domestic hot water (DHW) preparation. When supplied by district heating (DH), the operation of DHW tanks directly influences the DH return temperature, thereby affecting the DH system efficiency, supply capacity and the realization of the low temperature district heating. However, the conventional charging method often results in high DH return temperature. This study develops a new charging method aiming at reducing the DH return temperature without violating the comfort or hygiene requirements. The concept uses the multi-mode charging method considering the periodical characteristics of the load pattern. Multi scenarios are simulated by dynamic models using the practical DHW load profiles from a case study. Moreover, the impacts of the tank configuration, the location of the temperature sensor, and the distribution heat loss are investigated. The results show that the new control method can reduce the primary return temperature by 5–8 °C compared to the conventional control method. The distribution heat loss imposes great impact on the DH return temperature. In addition, the tank with the external heat exchanger performs better than the tank with the internal heating coil if the circulation heat loss is less than 50% of the DHW demand.

Gradient boosting machine for predicting return temperature of district heating system: A case study for residential buildings in Tianjin

Accurate prediction of the return temperature is critical to energy efficiency of the district heating system (DHS). The support vector machines (SVMs) and artificial neural networks (ANNs) have been widely applied to forecast energy consumption of the DHS recently. However, the parameters of SVM and ANN are difficult to be optimized due to their specific request for inputs and time-consuming training process. To explore the performance of the decision tree-based ensemble algorithms in the return temperature prediction task of the DHS, four return temperature prediction models based on the operational data of a heating system in Tianjin are established, namely Support Vector Regression (SVR), Multilayer perceptron (MLP), Random Forest (RF) and light gradient boosting machine (LGBM). The RF and LGBM are two typical decision tree-based ensemble algorithms. The historical supply temperature, outdoor temperature, relative humidity, wind speed and air quality index (AQI) are used as original inputs of the models. For computational models (SVR, MLP), the input features should be further transformed. The experimental results demonstrate that the LGBM model outperforms others in all standard evaluation measures. It shows that the tree-based ensemble models without complicated feature transformation achieves considerable results. Moreover, a week-based time series data splitting strategy is developed and compared with the traditional method. The experimental results show that the novel method can improve the performance of models except MLP. Overall, the performance of tree-based ensemble algorithms (RF and LGBM), SVR, and MLP are compared based on a case study in this article, illustrating the potential of the tree-based ensemble algorithms in thermal load prediction.

Demand side management in district heating systems by innovative control

Demand side management can be successfully applied to district heating systems for shaving thermal peaks. Peak shaving allows increasing share of convenient and less pollutant sources (waste heat, cogeneration and renewables) and enabling further building connections without modifying the pipelines. Demand side management in district heating is mainly done by shifting the load. Another interesting option consists in adjusting the substation regulation strategy; this approach not affects the heating schedule. This paper aims at analysing the opportunities for peak shaving using an innovative regulation strategy in the district heating substations, by controlling with a building model the effects on the indoor comfort conditions. The regulation strategy adopted is the Differential of Return Temperatures (DRT), that includes a constraint on the cold outlet section of the heat exchanger. This paper shows that thermal peak of building demand reducing on average of 15% by using the DRT regulation. Considering an entire distribution network, taking into account its thermal dynamics, the total peak request can be shaved of about 24%. Setting of the DRT regulation strategy has been shown being crucial for achieving satisfying peak shaving without compromising the indoor comfort conditions.

Benchmarking and Potential of Heat Pumps for Flue Gas Condensation

The use of environmental or waste heat with heat pumps, open absorption cycles or sorption heat pumps is an option for low carbon or high efficiency heat supply for industrial use. For one of the mentioned technologies to experience wide spread use it must offer economic advantages compared to other technologies. The evaluation of the economic viability is strongly dependent on boundary conditions.This work presents a method for the comparison of available technologies with regard to economics and potential for exhaust heat use. Calculations comparing the effects of different exhaust gas compositions resulting from technology specific air ratios for combustion in combination with different return and process heat temperatures are performed in order to quantify the potential of condensing technology. The herein developed specific annuity difference method allows evaluating the impact of improving technology specific parameters, such as temperature spread and coefficient of performance (COP), enabling to identify future research needs and to benchmark the technologies with e.g. a gas heater. The risk posed by uncertain future developments, such as gas price development and increasing prices for carbon emissions, possible taxation of these and emission trading, influences the economic evaluation and can motivate investment in active condensing technology, even if economic viability under current circumstances is not given.

Triple function substation and high-efficiency micro booster heat pump for Ultra Low Temperature District Heating

District-Heating (DH) covers around the 9% of the total heat demand in the EU, with a proven highperformance levels and low operational costs. DH may suffer adaptations in order to maintain competitiveness with individual heating systems. The most important one is the reduction of supply temperatures up to 50°C, emerging the concept of Ultra Low Temperature (ULT) DH. ULT DH allows the transmission and distribution heat losses minimization, since heat losses are proportional to the temperature-gradient between supply line the ambient, increasing overall system performance. Furthermore, enables the integration of low-grade energy sources with low marginal costs, such as solar thermal energy or waste heat from industrial and commercial buildings. This study presents the combination of a novel 3 function-scheme (3FS) substation and a micro booster heat pump for domestic hot water. The novel design of the substation allows different operation modes between the grid and the building according to the temperature level and demand range every moment. Regarding the microbooster, this unit is used directly to lift the temperature of the domestic hot water (DHW), so that risk of legionella is avoided and that the required comfort temperature is reached. Preliminary test for the energy performance of the booster heat pump was measured on serially produced units according to tapping profiles and methodologies of standard EN16147. DHW coefficients of performance of 5.2 and 8.5 were measured for heat source supply temperatures respectively of 25 and 40 °C and return temperatures of 22 and 30 °C. This paper explores the possible operational modes of a 3FS in combination with a Building Integrated Solar Thermal System (BISTS) and a Microbooster heat pump.

Integration of geothermal water into secondary network by absorption-heat-pump-assisted district heating substations

Integrating renewable energy into District Heating (DH) systems can significantly save the consumption of fossil energy and receive environmental benefits. The 4th Generation District Heating (4GDH) is the most promising technique for future DH systems by decreased supply temperature and use of low-temperature renewable thermal sources. As current district heating networks are mostly designed with supply temperature around 100°C, high hydraulic resistance and large pressure loss may often occur if decreasing the supply temperature to 30–70°C. This paper has proposed a Composite District Heating Substation (CDHS) with integrated geothermal water into the secondary network by Absorption Heat Pump (AHP). This has been demonstrated to be a feasible and effective solution for using low-temperature geothermal water in DH systems, without decreasing supply temperature of the primary network. The AHP is driven by hot water from the primary network, and the heat of the geothermal water could be used more intense by reducing its return temperature by the AHP and the plate heat exchanger. In this study, both the AHP and the whole substation were simulated by MATLAB, and the results indicated that the heating capacity of AHP increased when increasing the temperature or the mass flow rate of geothermal water, the mass flow rate of the primary network, or when decreasing the return temperature of the secondary network. Additionally, it was found that using temperature regulation with flow changing in stages in the secondary network could receive a higher COP of the AHP and a higher ratio of geothermal energy used in the substation. A case study was carried out as well and its results showed that comparing to conventional DH substations, the CDHS could save 8.0% of the annual cost and reduce 25.6% of the pollutant emissions for a heating season.

Improved Control of Radiator Heating Systems with Thermostatic Radiator Valves without Pre-Setting Function

Low-temperature district heating will play an important role in a future free of fossil fuels. This will only be able to be realized through the low-temperature operation of heating systems in existing buildings. Existing radiator systems can operate with low temperatures for most of the year because they are designed for extremely cold days, but errors have to be corrected and the control of the radiator systems needs to be improved. In this paper, we present a strategy to achieve low-temperature operation from the radiator system of a multi-family building in Denmark without a pre-setting function in the thermostatic radiator valves. The strategy is based on operating the system with a combination of a minimum supply temperature and small temperature differences over the radiators. The operation of the system is analyzed through a thermal-hydraulic model. A minimum supply temperature weather compensation curve was calculated and implemented in the central supply temperature control. Return temperature measurements in the substation, the risers, and several critical radiators were performed before and after the implementation of the strategy. The measurements confirm that a lower supply temperature results in a reduction of the return temperature. However, the system operator needs to be supported by a tool package to correctly maintain the system’s operation.

Experimental research of thermohydraulic dispatcher operating regimes

Heat carrier transportation in district heating (DH) systems is an issue of a significant energy saving potential. This potential can be achieved by applying a thermohydraulic dispatcher (THD) into DH systems. THD is a vertical or horizontal shunt pipe of a large diameter with relatively low hydraulic resistance in comparison with that of connected circuits. Installation of THD along with distributed variable speed pumps in DH systems can lead to reducing or even eliminating electric energy losses caused by throttling of redundant hydraulic head. It also leads to decreasing pressure in heat supply network which improves reliability of the network. But the opportunity of further rational implementation of THD in DH systems is limited because of insufficient amount of theoretical and experimental research. This paper is concerned with the experimental research of THD operating regimes. Already known aspects of THD operation were checked and proved. New dependencies were obtained for bypassing regime and for mixing regime of THD. Besides, different types of the primary circuit connection to THD were considered: connection for non-condensing boilers and connection for condensing boilers. Simplified models describing bypassing and mixing regimes of THD were proposed. It is possible to estimate return temperature of primary circuit with the help of the bypassing regime model with an error less than 1%. At the same time the mixing regime model is suitable for finding supply temperatures of primary circuits with an error less than 2%.The results of this paper can be used for further research and development of DH systems with THD. Such systems could be traditional as well as prospective ones (low temperature and pressure DH systems).

Achieving Lower District Heating Network Temperatures Using Feed-Forward MPC.

The focus of this work is to present the feasibility of lowering the supply and return temperatures of district heating networks in order to achieve energy savings through the implementation of feed-forward model predictive control. The current level of district heating technology dictates a need for higher supply temperatures, which is not the case when considering the future outlook. In part, this can be attributed to the fact that current networks are being controlled by operator experience and outdoor temperatures. The prospects of reducing network temperatures can be evaluated by developing a dynamic model of the process which can then be used for control purposes. Two scenarios are presented in this work, to not only evaluate a controller’s performance in supplying lower network temperatures, but to also assess the boundaries of the return temperature. In Scenario 1, the historical load is used as a feed-forward signal to the controller, and in Scenario 2, a load prediction model is used as the feed-forward signal. The findings for both scenarios suggest that the new control approach can lead to a load reduction of 12.5% and 13.7% respectively for the heat being supplied to the network. With the inclusion of predictions with increased accuracy on end-user demand and feed-back, the return temperature values can be better sustained, and can lead to a decrease in supply temperatures and an increase in energy savings on the production side.

Investigation of energy and exergy performances of radiant cooling systems in buildings – A design approach

Radiant cooling systems (wall and ceiling) are low exergy systems which can help in solving efficiently the comfort needs of occupants. However, a complex energy and exergy analysis should be done in the design phase of such cooling systems in order to choose the most appropriate solution. This paper attempts to present a mathematical model and methodology of such a complex analysis and illustrates through a case study the energy and exergy comparison of cooling systems with mechanical compression and absorption cooling systems. Simple relations were developed in order to evaluate rapidly the energy and exergy efficiency of these types of cooling systems taking into account the avoidance of surface condensation. It was proven that, besides the coefficient of performances of installed chillers, the temperatures in the generator and absorber, respectively the supply and return temperatures are the main influencing factors of the exergy efficiencies of such types cooling systems. Based on the developed mathematical method a numerical analysis was done assuming different operational parameters of the cooling system used in practice.

CFD analysis of triple-pipes for a district heating system with two simultaneous supply temperatures

Employing triple-pipes for simultaneous yet separate supply of space heating and domestic hot water demands of district heating network is an interesting solution for decreasing the rate of losses and increasing the cost-effectiveness of district heating systems. Although the thermodynamic performance of triple-pipes has been studied before, there is not any detailed performance analysis of such pipes, especially for the specific two-supply-temperature district heating design. This study employs computational fluid dynamic methods to investigate the feasibility of using triple-pipes for this district heating scheme and presents a detailed report of the thermal behavior of such pipes under different operational conditions including different pipe arrangements within the casing. For better understanding the effect of using triple-pipes in such a district heating system, the results obtained for this type of pipe is compared to those given for a twin-pipe in a regular district heating system. The results show that a triple-pipe with any arrangement is better than a twin-pipe supply method while this enhancement can be much better with a triangular arrangement of the three lines within the casing. With certain supply and return temperatures for a 10 km long pipeline, the rate of heat loss from the most efficient triple-pipe design is only 80 kW while it is about 145 kW for the twin-pipe system.

Trauma, Inflammation, and Anhedonia in Cocaine Use Disorder

The purpose of this paper is to provide an operation-based methodology for optimum district heating (DH) system design. The novelty is (1) analyzing long-term profiles of supply temperature, (2) considering actual DH operational data (3) considering both environmental and technical trends, (4) addressing the problems of detecting operational issues and proposing new design solutions. From the methodological point of view, the assessment methodology, that was originally defined in the statistical terms and which is able to quantify the number of temperature sojourns and quality of system management, have been implemented in the tool. Based on the temperature profiles and charts based on a statistical analysis, the flows and all supply and return temperatures may be forecasted for each actual temperature. The method indicates whether DH plant temperature control is appropriate or not; are any improvements required. The method is also able to show adjustments at the substation valves (secondary side). That means that we can analyze other DH systems and even show are they ready for transition to the 4GDH. To sum up, the main advantage of the developed method is the multiple focus on showing and proofing are any improvements of the network and control logic required.

Techno-Economic Design of Flue Gas Condensers for Medium-Scale Biomass Combustion Plants: Impact of Heat Demand and Return Temperature Variations

Despite their obvious benefit in terms of energy efficiency and their potential benefit on pollutant emissions, Flue Gas Condensers (FGCs) are still not widely spread in biomass combustion plants. Although their costs have significantly decreased during the last decade, the economic viability of FGC retrofits is not straightforward and their return on investments is mainly dependent on the temperature of the available heat sink and the moisture content of the fuel. Based on a new techno-economic model of a FGC validated with recent industrial data, this paper presents a methodology to assess the economic viability of an FGC retrofitting in a medium-scale biomass combustion plant. The proposed methodology is applied to the case of a typical District Heating plant for which real data was collected. For the first time, the usual assumptions of constant process data generally used are challenged by considering the variability of the return temperature and heat demand over the year. Furthermore, a new concept of optimal configurations in terms of energy savings is introduced in this paper and compared to a strictly economic optimum. The economic feasibility is mainly evaluated by means of the Net Present Value (NPV), Discounted Payback Period (DPP), and the Modified Internal Rate of Return (MIRR). As expected, results show that the higher the humidity level and the lower the return temperature, the higher the economic profitability of a project. The NPV is, however, increased when considering variable inputs: Even with an average return temperature of 60 °C, a mixed operation of the FGC as a condenser and an economizer along the year is predicted, which results in an increased profitability assessment. Considering a constant return temperature over the year can lead to a 20% underestimation of the project NPV. An alternative averaging method is proposed, where two distinct temperature zones are considered: above and below the flue gas dew point. The discrepancy with a detailed temperature variation is reduced to a few percents. Our results also show that increasing the FGC surface beyond the highest NPV can lead to substantial energy savings at a reasonable cost, up to a certain level. The energetic optimum we defined can lead to an increase in energy savings by 17% for the same relative decrease of the NPV.

Analyzing the Performance and Control of a Hydronic Pavement System in a District Heating Network

A hydronic pavement system (HPS) is an alternative method to clear snow and ice, which avoids the use of salt, sand, and fossil fuel in conventional snow clearance, and minimizes the risk of accidents. The aim is to analyze the performance of different control strategies for a 35,000 m2 HPS utilizing heat from a district heating and cooling (DHC) system. The key performance indicators are (1) energy performance of the HPS, and (2) primary energy use, (3) electricity production and (4) greenhouse gas (GHG) emissions from the DHC system. The methodology uses a simulation model of the HPS and an optimization model of the DHC system. Three operational strategies are analyzed: A reference scenario based on the current control strategy, and scenarios where the HPS is shut down at temperatures below −10 °C and −5 °C. The study shows that the DHC return temperature is suitable for use. By operational strategies, use during peak demand in the DHC system can be avoided, resulting in reduced use of fossil fuel. Moreover, the energy use of the HPS could be reduced by 10% and the local GHG emissions by 25%. The study emphasizes that the HPS may have positive effects on global GHG emissions, as it enables electricity production from renewable resources.

Faults in district heating customer installations and ways to approach them: Experiences from Swedish utilities

The district heating (DH) customer installations in current DH systems contain a variety of different faults that cause the return temperatures of the systems to increase. This is a major problem, since the focus in the DH sector is to decrease the system temperatures in order to utilize more low-temperature heat. Therefore, this study has focused on how utilities are currently working to keep their temperatures low, how they involve their customers in this work, and what faults are most common today. This was done by conducting a combined interview and survey study, where Swedish DH utilities participated. The results showed that the two most important elements to obtain low return temperatures were to have physical access to and mandate of the customer installations, and to maintain a good and close customer relationship. The results also showed that many faults occur in the customers’ internal heating systems, or were due to leakages somewhere in the installation. Overall, the results showed that it is indeed possible to work close to and affect the customers to obtain lower return temperatures from the customer installations. It was also clear that the most common faults were rather easy to eliminate as long as the utilities gained physical access to the entire customer installation.

Mathematical Analysis of Optimal Operating Conditions in Heating Systems

With changes in the outdoor air temperature, the heat consumption of buildings also changes. Timely adjustment of the heating systems to ensure optimal operating conditions is extremely significant to save energy. In this study, the operation conditions of a heating system were analyzed numerically, and the existence, uniqueness, and stability of the optimal operation conditions of the heating system were proved. An operation optimization model that could obtain the optimal operation conditions was also established, and the correctness of the model was verified experimentally. Experimental results showed that when the flow rate was 0.606 m3/h, the supply water temperature was 67.13°C, water return temperature was 65.90°C, and the pump consumed the least amount of electricity. The experimental results and model calculation results showed that the operating cost is lower when the system flow rate is low and the supply water temperature is high under the same heat dissipation and indoor temperature.